The present invention relates to ophthalmic filter materials. These materials selectively transmit radiation in the visual and near-visual regions of the spectrum. The present materials are primarily concerned with controlled transmission of sunlight through ophthalmic lenses.
The invention was developed employing photochromic glass lenses. However, for some purposes, photochromic glass is not suitable because of safety and/or weight. Fortunately, the invention is also applicable to tintable plastic materials as well. The invention may also find application in products other than glass and plastic, ophthalmic lenses, such as visors and shields.
The invention was developed using photochromic glass lenses, and is largely so described. However, as indicated, the invention is not so limited, and may also be practiced employing tintable plastic materials. Accordingly, that practice is also described.
All of the commercially important photochromic glasses contain a precipitated, microcrystalline, silver halide phase. It is this phase which is considered to cause the reversible darkening of the glass under exposure to light. U.S. Pat. No. 3,208,860 (Armistead et al.) provides the basic description of this family of glasses.
The patent is drawn generally to silicate-based glasses. The preferred compositions are encompassed within the alkali metal oxide-Al.sub.2 O.sub.3 --B.sub.2 O.sub.3 --SiO.sub.2 system. Thus, the preferred base compositions consist essentially, expressed in weight percent on the oxide basis, of about 4-26% Al.sub.2 O.sub.3, 4-26% B.sub.2 O.sub.3, and 40-76% SiO.sub.2, wherein R.sub.2 O is selected from the group of 2-8% Li.sub.2 O, 4-15% Na.sub.2 O, 6-20% K.sub.2 O, 8-25% Rb.sub.2 O, and 10-30% Cs.sub.2 O, the sum of the base glass ingredients constituting at least 85% of the total composition. The patent further observes the advantage of including small quantities of low temperature reducing agents, such as SnO, FeO, CuO, As.sub.2 O.sub.3, and Sb.sub.2 O.sub.3, to improve the photochromic characteristics of the glass. In order to impart preferred photochromic behavior to the glass, a silver halide is present in the glass.
Subsequent work has resulted in the development of a number of specific families of photochromic glasses exhibiting faster darkening and/or fading response. U.S. Pat. No. 4,190,451 (Hares et al.), for example, provides a description of more recently developed photochromic glasses of this type. The glasses described in that patent consist essentially, in weight percent, of about 0-2.5% Li.sub.2 O, 0-9% Na.sub.2 O, 0-17% K.sub.2 O, 0-6% Cs.sub.2 O, 8-20% Li.sub.2 O+Na.sub.2 O+K.sub.2 O+Cs.sub.2 O, 14-23% B.sub.2 O.sub.3, 5-25% Al.sub.2 O.sub.3, 0-25% P.sub.2 O.sub.5, 20-65% SiO.sub.2, 0.004-0.02% CuO, 0.15-0.3% Ag, 0.1-0.25% Cl, and 0.1-0.2% Br, wherein the molar ratio of alkali metal oxides:B.sub.2 O.sub.3 ranges about 0.55-0.85, and the weight ratio Ag:(Cl+Br) ranges about 0.65-0.95. The glasses may also contain up to about 10% total of optional constituents, expressly referring to 0-6% ZrO.sub.2, 0-3% TiO.sub.2, 0-0.5% PbO, 0-7% BaO, 0-4% CaO, 0-3% MgO, 0-6% Nb.sub.2 O.sub.5, 0-4% La.sub.2 O.sub.3 and 0-2% F. Also, up to 1% total of transition metal oxide colorants and/or up to 5% total of rare earth metal oxide colorants can be included without adversely affecting the photochromic behavior of the glass.
U.S. Pat. No. 4,284,686 (Wedding) explains in some detail that certain diseases of the eye and/or visual deficiencies may be caused by, or may be aggravated by, strong sunlight. The patent notes that radiation at the short end of the visible spectrum, that is, at wavelengths on the order of 400-550 nanometers (nm), seems to create the greatest problems for some patients.
The patent observes that the eye contains two different kinds of photoreceptors, viz., cones and rods. The cones comprise the principal receptors in daylight vision (photopic vision); the rods constitute the principal receptors in night vision (scotopic vision).
The cones are located generally in the central portion of the retina, and are much greater in number/unit area. They permit the recognition of fine detail, presumably because they function largely independently of one another. The cones also permit color vision, i.e., they allow hues and saturation to be distinguished. In the presence of bright light, the eye is most sensitive to radiation at about 555 nm.
In general, the rods are located in the peripheral portions of the retina, there being few, if any, rods found in the central retina. The rods do not permit the recognition of colors, only shades of gray. Their peak wavelength sensitivity is near 510 nm.
As a corrective measure, the Wedding patent provides photochromic glasses that are treated to produce a colored surface layer. The colored surface layer exhibits near-zero transmittance of radiations having wavelengths shorter than a selected cutoff wavelength in the range 440-550 nm. The photochromic feature permits the same spectacles to be used both indoors and outside. To develop the colored layer, the photochromic glass is heated in a strongly reducing atmosphere, such as hydrogen. The exposure is at temperatures in the range of 350-520.degree. C. for at least 12 hours.
Commercial glass lenses, developed on the basis of the patent teaching, have proven very useful for patients having relatively strong sensitivity to sunlight. However, the lenses generally have a yellow to orange color. From a cosmetic standpoint, this is regarded by many potential users to be quite unattractive.
Dye-impregnated plastic lenses have been developed as alternatives. The latter are sometimes referred to as "blockers", since they are stated to absorb all of the light below a certain wavelength. A major problem with the "blocker" lens is that total absorption of part of the spectrum greatly distorts color perception. This may also occur in the surface colored glass lens to some extent.
U.S. Pat. No. 5,381,193 (Wedding) describes an ophthalmic, protective filter lens having a scotopic transmittance (Y.sub.S) lower than the photopic transmittance (Y.sub.P) by more than a factor of two. The filter lens has a dominant wavelength within the range of 580-605 nm and a color purity in the range of 75-85%. Thereby, colors perceived through the lens are somewhat distorted from those of the scene being viewed. In an embodiment, a lens is composed of a photochromic glass having a coloration developed in at least one surface.
Ophthalmic lenses were developed on the basis of the patent teaching to cope with nocturnal vision problems. The lenses were designed to facilitate recovery of nocturnal vision after prolonged exposure to intense sunlight such as encountered in a desert.
Subsequently, the lenses were tested for use by persons beginning to experience a high sensitivity to bright sunlight under normal conditions. Criticism was encountered because the preferred embodiment for nocturnal vision exhibited an olive-green cast in the faded state. While the lenses of the -193 patent were designed to provide improved natural color perception relative to the lenses of the -686 patent, a desire for even less color distortion was expressed.
It is a basic purpose of the instant invention to provide an ophthalmic filter lens that meets these criticisms. A particular purpose is to provide a photochromic filter lens designed for outdoor use. Another purpose is to provide such a filter lens having an improved cosmetic aesthetic appeal. A further purpose is to provide a filter lens that combines specific features of protective lenses developed in accordance with the teachings of the Wedding -686 patent. Another purpose is to provide the filter benefits of the invention in a progressive lens.